Aerial devices are used to carry out a number of different types of jobs, including many types of work in the electrical utility industry. The aerial device includes a vehicle mounted boom having a personnel carrying platform on its end. The boom can rotate on the vehicle bed and can pivot up and down in order to position the platform advantageously for the work that is to be done. The boom typically includes multiple sections which can be connected for articulation about an elbow joint or for telescopic extension and retraction.
A common use of aerial devices in the electric utility industry involves work in close proximity to overhead power lines which are energized at high voltage levels. For safety, the boom is typically constructed of fiberglass in order to electrically isolate the platform area from the ground. On multiple section booms such as an articulating boom assembly having upper and lower booms, the upper boom is commonly constructed of fiberglass and the lower boom commonly has a fiberglass insert. The base end of the boom at the vehicle chassis, the elbow joint between the two booms, and the platform area all have metal components that are electrically conductive. The chassis can be grounded to keep the base end of the boom safety in a grounded condition. The insulated booms serve to isolate the elbow joint and particularly the platform from the ground so that a circuit path to ground is not presented along the boom.
Under prevailing industry standards, workers at the platform working on power lines at 47 Kv or less are considered to be primarily protected by wearing insulated rubber gloves or using insulated "hot sticks" in conjunction with work practices that incorporate minimum recommended approach distances to the power lines. The insulated boom is considered to be secondary or back up protection for these workers. However, as a practical matter, workers do not always follow these recognized safety practices.
If standard safety practices are not followed, there are several ways in which personnel in an aerial platform can be exposed to electrical dangers. For example, a worker who is not wearing gloves or using a hot stick can contact two different electrical phases or one phase and a grounded object, thus completing a circuit through his body. This type of incident does not involve the aerial device at all and can be avoided through the use of proper safety techniques.
If the boom should lose the dielectric integrity which provides its insulating properties, a conductive path is created from the platform through the boom to the ground, thus endangering personnel on the platform as well as those on the ground. Inspection and maintenance practices and periodic testing are currently used to minimize the likelihood of the boom loosing its insulating properties.
Under some conditions, the platform can loose its electrical isolation from the ground even though the boom insulation is effective. There are a number of metal components typically present on the platform, including control handles for the hydraulic valves and various metal brackets and other hardware. Any of the metal components can unintentionally come into contact with an energized conductor or a grounded object such as a ground wire, a guy wire, or even a pole or tree. If a worker who is not wearing gloves and who touches a grounded object happens at the same time to contact a metal part that is in contact with an energized power line, a circuit is completed through his body without any platform movement. Similarly, even when the platform is stationary, an unprotected worker may grasp an energized conductor and inadvertently touch a metal part that is in contact with a grounded object.
A different type of hazardous situation results when an operator moves the platform while he is in contact with an energized conductor or a grounded object, and the platform moves to where a grounded object or a power line contacts a metal part of the boom tip such as a control handle the worker is using. Again, a circuit path is completed through the body of the worker, this time as a result of platform movement
Cranes and similar devices have been equipped with alarm systems to warn against electrical hazards. Typically, these devices use proximity sensors to detect when the boom structure is near a power line. Some systems even deactivate the boom if the electrical field exceeds a predetermined level. This type of equipment is constructed using conductive booms, so if a power line is contacted, it is too late to take corrective action because a ground path has already been created through the boom. Consequently, the warning system gives an alarm when a power line is being approached and before it is actually contacted. This type of alarm system is useless with an aerial device because the intent is to work in close proximity to energize power lines. No purpose is served by providing a warning signal because the workers already know they are near power lines and they intend to be there.